This invention relates to a process for producing acetone and more particularly it relates to a process for producing acetone by oxidizing propylene by means of an oxygen complex.
Acetone is a representative aliphatic ketone and has been used not only commercially used as solvents for acetylcellulose, nitrocellulose, acetylene, etc. in a large amount, but also used as solvents for fats, resins, camphor, etc. in pharmaceutical field, and further as synthetic raw materials for many intermediate products such as methyl isobutyl ketone, methyl isobutyl carbinol, methyl methacrylate, bisphenol A and further ketene, etc.; thus it has occupied an important position in chemical industries.
The commercial process for producing acetone is roughly classified into the following ones: (1) isopropanol dehydrogenation process, (2) cumene process and (3) propylene oxidization process.
Isopropanol dehydrogenation process among these has been operated under severe conditions of 300.degree..about.500.degree. C. and 3 atm in the presence of a dehydrogenation catalyst such as ZnO, Cu, etc. and the isopropanol conversion and the selectivity to acetone have been regarded as 98% and 90%, respectively (see Industrial Organic Chemistry, page 266, written by K. Weissermel and H. J. Arpe and translated by Teruaki Mukaiyama, Tokyo Kagaku Dojin (1978)). Further, cumene process is directed to a process wherein cumene is oxygen-oxidized at 120.degree. C. and under the atmospheric pressure in liquid phase in the presence of Cu, Co salt or the like as catalyst to prepare cumene hydroperoxide, which is then decomposed with 0.1.about.2% H.sub.2 SO.sub.4 at 60.degree..about.65.degree. C. into phenol and acetone. The selectivity based on cumene is about 90% and the proportion of the products is 60% of phenol and 40% of acetone. These processes have been currently operated, but a process for producing acetone under milder reaction conditions and at a single stage has been noted, that is, the so-called Wacker process wherein propylene is used as raw material and palladium chloride (Pd(2)Cl.sub.2)-cupric chloride (Cu(2)Cl.sub.2) is employed as catalyst. This process is most characteristic among processes for producing acetone (see the above literature, page 265). According to this process, a composite catalyst obtained by dissolving Pd(2)Cl.sub.2 and Cu(2)Cl.sub.2 as catalyst in a hydrochloric acid solution (pH: 0.about.2) is employed. First propylene is oxidized by divalent palladium (Pd(2)) and water to form acetone (CH.sub.3 COCH.sub.3). Water participates in this reaction as shown in the following equation: ##STR1##
As seen from this reaction equation, Pd(2) is reduced into metal palladium (Pd(0)) which precipitates. It is necessary to prevent this and also oxidize Pd(0) into Pd(2) for regeneration by making a large amount of Cu(2)Cl.sub.2 coexistent therewith, as shown in the following equation: EQU Pd(0)+2Cu(2)Cl.sub.2 .fwdarw.Pd(2)Cl.sub.2 +2Cu(1)Cl (2)
Further, difficultly soluble Cu(1)Cl byproduced at that time is oxygen-oxidized in the coexistence of HCl according to the following equation and returned to Cu(2)Cl.sub.2 : EQU 2Cu(1)Cl+1/2O.sub.2 +2HCl.fwdarw.2CuCl.sub.2 +H.sub.2 O (3)
As described above, by employing a redox system of Pd(2)/Pd(0) and Cu(2)/Cu(1), a continuous oxidation of propylene is made possible. However, since the oxygen molecule is not directly reacted with propylene, but a complicated oxidation-reduction reaction of the Pd(2)/Pd(0)--Cu(2)/Cu(1) system is utilized as described above, this constitutes a rate-determining step of the reaction. Further, since difficultly soluble Pd(0) and Cu(1)Cl are formed midway the reaction, a high concentration of HCl aqueous solution (pH: 0.about.2) is used and hence choice of a corrosion-resistant material is required. Still further, since the solubility of oxygen in water is low, its solubility must be raised, and since the higher the olefin, the lower the reactivity, the reaction conditions are as severe as 140.degree. C. and 14 atm. In addition, it has been regarded that under such conditions, the conversion of propylene is 99% or higher, the selectivity to acetone is 92% and 2.about.4% of propionaldehyde is produced (see the above literature, page 265). Furthermore, if oxygen dissolved in excess is discharged into the gas phase, it is mixed with propylene and has a possibility of troubles such as explosion; hence a countermeasure thereto becomes necessary (Revised complete works of production flow sheet, edited by Kihara et al, Vol. II, p. 296, Kagaku Kogyosha (1978)).
As described above, any process has been carried out at relatively high temperatures and pressures; thus a process wherein acetone can be produced under milder conditions and at a single stage has been desired.